1. Field of the Invention
The present invention relates to a fan motor having two-step speed control, and more particularly to a fan motor that may output smooth voltage waveforms by a buffer circuit while altering the voltage level, such that the speed of the fan is increased or reduced gradually when the fan is switched between a low speed operation mode and a high speed operation mode, thereby lengthening longevity of the fan motor.
2. Description of the Related Art
It is common to employ heat-dissipating fans in computers and their peripherals to avoid break down of the system or burnt of the electronic elements resulting from temperature rise due to poor heat dissipation. During operation of a fan motor, the fan motor is operated in a low speed mode when the temperature in the system is not high. In this case, unnecessary load and noise are generated if the fan motor is operated at high speed. When the system is under full load and thus has a relatively high temperature that requires the fan operates in a high speed mode, otherwise the system may break down and/or the electronic elements may be burnt. In order to solve the above problems, speed-controllable heat-dissipating fans have been developed to adjust the speed of the fan in response to different system temperatures.
A conventional fan motor is shown in FIG. 1 and FIG. 2 (including FIGS. 2A-2C) of the drawings that correspond to FIG. 2 and FIG. 5 (including FIGS. 5A-5C) of U.S. Pat. No. 5,197,858 to Cheng issued on Mar. 30, 1993. FIG. 1 is a circuit diagram of a controller for the fan. FIG. 2 illustrates the output waveforms for the drive IC of the circuit. As illustrated in FIG. 1, when the power is on, via an reverse voltage protection diode D1, impellers start to rotate by mutual induction between winding coils and magnet. At this time, a Hall element IC1 senses the variation of magnetic field between winding and magnet to cause the DC brushless motor to commute as follows: A predetermined current and DC level are supplied by resistors R3, R2. Positive (V+) and negative (Vxe2x88x92) voltages are both output from the Hall element IC1 to a driving integrated circuit IC2. The two voltage waveforms can be processed and shaped by means of the driving integrated circuit IC2 by comparing them with an internal voltage to obtain the waveform shown in FIG. 2A. This waveform controls semiconductor switches A1 and A2 to obtain the waveform shown as FIGS. 2B and 2C. Motor windings L1, L2, L3, and L4 are controlled by the wave output from the semiconductor switches A1, A2 to commutate in accordance with the magnetic couple of impeller ring-shaped magnet. The timing capacitor provides voltage to the driving integrated circuit IC2 for re-starting of the motor from a completely stop state of the fan. As a result, a driving system composed of IC1 and IC2 can drive the fan and output a cycle-timing pulse signal.
IC3 comprises three internal operational amplifiers IC31, IC32. IC33. Operational amplifiers IC31, with resistors R4, R5, R6, R7, R8, R9, R10 and a thermal sensor Rth in combination, forms a control circuit for the slope of the curve of the speed versus the temperature of the thermal control variable speed fan. Because the resistance value of the thermal sensor Rth changes with temperature, the voltage Va which is dependent upon the resistance of sensor Rth and resistor R4 will also be changed as the temperature changes. Voltage Va and the reference voltage Vref, which is controlled by the voltage divider formed by resistors R9 and R10, are input into operational amplifier IC31, to obtain a variable voltage Vb, which causes the collector current of transistor TR1 to change accordingly, changing the speed of the fan. Therefore, the object of the variable speed by thermal control is achieved.
Nevertheless, the waveforms output from the drive integrated circuit IC2 to the windings L1, L2, L3, and L4 are square waveforms, as shown in FIGS. 2B and 2C. In addition, although the change in the output voltage Vb by the operational amplifier IC31 in response to change in the system environmental temperature make a change in the conductive current in the transistor TR2, output waveforms of the transistor TR2 are still square waveforms. Thus, rotating speed of the system fan is increased or reduced dramatically due to square waveforms inputted to the windings L1, L2, L3, and L4. As a result, the fan wobbles and thus has a shortened longevity.
Another conventional fan motor is shown in FIGS. 3 and 4 of the drawings that correspond to FIG. 2 and FIG. 3 of U.S. Pat. No. 5,942,866 to Hsieh issued on Aug. 24, 1999. FIG. 3 is a schematic block diagram of a control circuit. FIG. 4 shows the waveform signal outputted from a switching device of the control circuit. As illustrated in FIG. 3, a control circuit 10 for a DC brushless fan comprises a rectifying circuit 20, a comparator 21, and a switching device 22. The rectifying circuit 20 receives a continuous, square wave signal from the fan 23, which is indicative of the rotating speed of the fan 23, and then sends a rectified and filtered DC voltage signal V1 to inverted input terminal of the comparator 20. The non-inverted input terminal of the comparator 21 is connected to a reference voltage signal Vref, which is used for setting the rotating speed of the fan 23, and the output terminal of the comparator 21 is connected to the switching device 22. The switching device 22 may be a transistor or an equivalent electronic switch that is serially connected between a source voltage Vcc and the source terminal of the fan 23. The operation of the switching device 22 depends on the compared result of the rectified DC voltage signal V1 outputted from the rectifying circuit 21 and the reference voltage signal Vref.
When the DC voltage signal V1 outputted from the rectifying circuit 21 is lower than the reference voltage signal Vref, i.e., the rotating speed of the fan 23 is lower than its setting value, the comparator 21 outputs a Logic high value to the switching device 22. Then, the switching device 22 is closed, and the fan 23 is powered on. Thus, rotating speed of the fan 23 will be increased.
In contrast, when the DC voltage signal outputted from the rectifying circuit 20 is higher than the reference voltage signal Vref, i.e., the rotating speed of the fan 23 is higher than its setting value, the comparator 21 outputs a Logic low value to the switching device 22. Then, the switching device 22 is opened, and the fan 23 is powered off. Thus, rotating speed of the fan 23 will be decreased.
In operation, the switching device 22 is repeatedly closed and opened as the rotating speed of the fan varies, thus the fan is intermittently powered on, whereby the rotating speed of the fan 23 can be controlled and kept at a constant value. As illustrated in FIG. 4, the output signal of the switching device 22 is an intermittently opened and closed square wave, where the period (TIME ON) during which the switching device 22 is closed and the period (TIME OFF) during which the switching device 22 is opened are modulated so as to control the rotating speed of the fan 23.
Nevertheless, the output waveform is an intermittently opened and closed square waveform, and the rotating speed of the fan 23 is increased or decreased dramatically in response to opening or closing of the sequare waveform or the switching device 22. As a result, the fan wobbles and thus has a shortened longevity.
In view of the above drawbacks, the present invention provides a buffer circuit that is connected to the voltage source. The buffer circuit outputs different voltages in response to a change in the control voltage for driving the fan. During a change between a high voltage value and a low voltage value, the voltage waveform is a smooth linear waveform. Rotating speed of the fan is increased or decreased gradually to avoid dramatic change in the rotating speed of the fan to thereby lengthening longevity of the fan.
It is the primary object of the present invention to provide a fan motor having two-step speed control, wherein a buffer circuit is provided between the voltage source and a source terminal of the fan. The buffer circuit outputs a smooth linear voltage-varying waveform. Rotating speed of the fan is increased or decreased gradually to avoid dramatic change in the rotating speed of the fan to thereby lengthening longevity of the fan.
It is another object of the present invention to provide a fan motor having two-step speed control with less and simple component and by means of a resistor resistance to set the low speed of the fan. The fan motor mainly comprises a buffer circuit, a control voltage terminal, and a fan. The buffer circuit is mounted between the voltage source and a source terminal of the fan. The buffer circuit comprises a resistor and a transistor, wherein the control voltage terminal is connected to the base of the transistor of the buffer circuit. When the voltage of the voltage source equals to the control voltage, the transistor of the buffer circuit is open. At this time, the resistor of the buffer circuit will divide the source voltage to lower the fan-driving voltage, and the fan operates at the low-speed operation mode. When the control voltage is zero, the transistor of the buffer circuit is closed such that the fan-driving voltage equals to the voltage of the voltage source, and the fan operates at the high-speed operation mode.
Other objects, specific advantages, and novel features of the invention will become more apparent from the following detailed description and preferable embodiments when taken in conjunction with the accompanying drawings.